Fused rebonded basic brick

ABSTRACT

CERAMICALLY BONDED BASIC BRICK MADE FROM A BATCH COMPRISING AT LEAST TWO CHEMICALLY DIFFERENT FUSED MAGNESITE-CHROME ORE GRAINS BEING IN DIFFERENT SIZE FRACTIONS OF THE BATCH.

United States Patent O 3,578,474 FUSE!) REBONDED BASIC BRICK Dwight S. Whittemore, Bethel Park, Pa., assiguor to Dresser Industries, Inc., Dallas, Tex. N Drawing. Filed Oct. 3, 1967, Ser. No. 672,455 Int. Cl. C0411 35/42 US. Cl. 106-59 7 Claims ABSTRACT OF THE DISCLOSURE Ceramically bonded basic brick made from a batch comprising at least two chemically different fused magnesite-chrome ore grains being in different size fractions of the batch.

BACKGROUND Ceramically bonded or burned brick are treated at elevated temperatures during which sintering and coalescing of fine particles results in the formation of a ceramic bond. Fused cast refractories are manufactured by casting a molten refractory into molds and carefully cooling and annealing. Fused, rebonded refractories are manufactured by fusing a raw materials batch, cooling the fusion in any convenient manner, size grading the fusion, and thereafter pressing into shapes and burning to obtain a ceramic bond. Fused, rebonded brick have been found to have many of the properties of fused cast refractories while being substantially less dllficlllt to manufacture. This invention pertains to improved fused, rebonded brick.

This invention pertains to magnesite-chrome and chrome ore-magnesite brick. Magnesite-chrome brick are those prepared from a batch comprising dead burned magnesite (or any other source of magnesia) and chrome ore in which the dead burned magnesite or equivalent is predominant. Chrome-magnesite brick are prepared from batches in which the chrome ore is predominant.

High temperature tensile strength (measured by modulus of rupture at 2300 F.) of fused, rebonded chrome-magnesite and magnesite-chrome brick is dependent on the magnesitezchrome ore ratio of the brick. Generally, the larger the amount of chrome ore in the brick the greater the high temperature tensile strength. Unfortunately, as the chrome ore in the batch is increased, the modulus of elasticity also increases resulting in reduced thermal spalling resistance.

It is an object of this invention to provide a method of making fused, rebonded magnesite-chrome ore brick with improved high temperature tensile strength and improved resistance to thermal spalling.

It is another object of this invention to provide improved fused, rebonded magnesite-chrome brick.

BRIEF DESCRIPTION OF THE INVENTION According to this invention, ceramically bonded brick are made from a size-graded batch comprised of at least two different types of magnesite-chrome ore grains. The batch is sized to have a coarse (pus 65 mesh) and a fine (minus 65 mesh) fraction. The fine fraction and the coarse fraction are comprised of fused magnesite-chrome one grains having substantially different magnesite: chrome ore ratios. It is preferable that the chrome ore content of the fines fraction is at least 10 percent different than the chrome ore content of the coarse fraction. Preferably, if the magnesitezchrome ore ratio of the coarse fraction is in excess of 55:45, then the magnesitezchrome ore ratio fine fraction is below 45:55. (The magnesite: chrome ore ratio is the weight ratio of the magnesia conice tributing ingredient on a calcined basis to chrome ore ingredient. By calcined basis is meant the weight of the magnesia ingredient if it would be dead burned.) It is also permissible to reverse the placement of the fused grain such that the fused grains having a magnesitezchrome ore ratio in excess of 55:45 are in the fine fraction. The magnesite-chrome ore grains must be blended such that the over-all MgO content of the brick lies between about 55 and percent, preferably between about 60 and 70%. (Both dead burned magnesite and chrome ore contribute magnesia to the brick.) Brick made according to the above description have improved high temperature tensile strength and thermal spall resistance.

DETAILED DESCRIPTION Further features and other objects of the invention will become clearer to those skilled in the art by careful study of the following detailed description. In the specification and claims all percentages and ratios and parts are by weight; chemical analyses were obtained by spectrographic analysis and controlled by wet chemical analysis, and are reported as oxide in accordance with the present practice of the refractories industry. All sizing are measured with Tyler Standard Screens Scale Sieve series.

Three fused magnesite-chrome ore grains were prepared by blending magnesia and Transvaal chrome ore in the ratio :20, 60:40, and 40:60. Typical chemical analysis of the raw materials used in the preparation of the fused grain is given in Table I.

It is suitable according to this invention that any refractory chrome ore be used so long as it has a silica content of less than about 3 percent. Any source of magnesia is suitable so long as it has a silica content of less than 3 percent. The silica content of the fused magnesitechrome ore grain should be less than 3 and preferably less than 2 percent. The grains are fused in electric arc furnaces in which they are rendered almost totally molten before being allowed to cool and resolidify.

The above described fused magnesite-chrome ore grains were sized-graded into three size fractions used in brickmaking, namely 4 on 10 mesh, 10 on 28 mesh and ball milled fines which are substantially all minus 65 mesh and typically 30 to 50 percent passing 325 mesh. The size fractions of the various fused grains were combined for the preparation of Examples A, B, C, and D as shown in Table II. It should be understood that the size-grading used in the preparation of the exemplary mixes is not the only size-grading that can be used according to the teachings of this invention. Those skilled in refractories art realize that a dense refractory brick can be prepared from a batch which has a minus 65 mesh fraction constituting from 25 to 50 percent of the batch and a plus 28 mesh fraction consisting of at least 40 percent of the batch by weight.

TABLE II Percent Example Fused 80% magnesite-20% chrome ore grain:

4 10 meslL...

The batches described in Table II were blended and tempered with a temporary binder, in this case a lignosulfonate liquor to form a pressable batch. The batches were pressed into brick at about 8000 p.s.i., dried overnight at about 250 F., and burned with a 10 hour hold at a maximum temperature of 3150 F. The brick made according to Examples A, B, C, and D were then subjected to a typical test for refractories brick. The results of these tests, along with the chemical analyses of the brick according to the examples, are given in Table III.

TABLE III Examples A B C D Bulk density (ASTM test 013441),

p.c.f 200 199 207 203 Modulus of rupture, p.s.i.:

At room temperature 1, 150 1, 000 1, 430 1, 340 At 2,300 F. (hold time 5 hrs.) 2, 600 2, 060 2, 780 2, 190 At 2,700 F. (hold time 5 hrs.)

(ASIM test 0133-55) 1, 240 860 1, 210 640 Apparent porosity (avg. 3) (ASTM test 020-46), percent 16. 6 15.9 16. 1 Modulnq of elasticity (X10 p.s.i.) (avg.

3) (same method) 3. 21 2. 93 4. 26 4. 05 Loss in panel spalling test with preheat at 3,000 F., percent 10. 1 INo 36. 9 23. 1

oss Spectrochemical analysis, percent:

Silica (SlQz) 0.5 0.50 0.77 1.0 Alumina (A1203) 6. 9 5. 1 8.2 6. 7 Titania (TiOz) 0 21 0. 0. 29 0. 24 Iron oxide (F6203) 11. 0 9. 3 12. 3 11. 6 Chromic oxide (CD03) 21. 6 15. 6 25. 2 18. 1 Lime (CaO) 0. 30 0 0. 0.70 Magnesia (MgO) 59. O 69. 5 53. 4 61. 1

Brick made according to Examples A and B are according to the teachings of this invention. Example B is the best mode now known for the practice of this invention. Table III establishes that brick made according to this invention have both improved high temperature tensile strength and improved resistance to thermal spalling. (Compare brick according to Examples A and D which have almost identical chemical analyses.) In example D only one type of grain was used in the brick-making batch. Notice brick according to Example A had better hot tensile strength and better thermal spall resistance.

Brick made according to Example A are prepared from brick-making batches in which the fine fraction comprises r fused grain having a high magnesitezchrome ore ratio and the coarse fraction comprises fused grain having a low ratio. On the other hand, brick made according to Example B are made from batches in which the coarse fraction comprises fused grain having a high magnesite: chrome ore ratio and the fine fraction comprises fused grain having a low ratio. Both are according to the teachings of this invention. However, Table III establishes that it is preferable where high temperature tensile strength, is more critical than thermal spalling resistance to place the higher magnesite grain in the fine fraction. On the other hand, if thermal spalling resistance is more critical, it is preferable to place the higher magnesite grain in the coarse fraction.

One of the features of this invention is maintaining the overall magnesia content of the brick within certain limits, that is, between 55 and 75 percent, by weight of the brick.

Brick made according to Example C have a magnesite content that is too low and, as a result, have poor thermal shock resistance.

While the scientific principles upon which this invention is based are not entirely understood, it is believed that providing the brickmaking batch with fused grains having different magnesia chrome ore ratios creates chemical gradients which, during the burning process, promote fusion and thereby promote sintering and coalescing which are necessary for good high temperature tensile strength. The combination of grains having different magnesia: chrome ore ratios in the brickmaking batch also provides the brick with lower moduli of elasticity. It is well known in the refractories art that thermal spalling resistance is very directly related to modulus of elasticity. Brick having lower moduli of elasticity have ability to accommodate strains built up by rapidly changing the thermal gradients. Generally, when high temperature tensile strength is increased as by increasing the percentage chrome ore in fused, rebonded magnesite-chrome ore brick, the modulus of elasticity is also increased thereby resulting in brick of poor thermal spall resistance. However, according to this invention, brick are provided with increased high tensile strength and increased thermal spall resistance.

It should be understood that the advantages of this invention are realized in brick made other than by pressing; for example, by slip casting or ramming or extruding. It should also be understood that while firing temperatures of 3150 F. are preferable, temperatures in excess of 2800 F. are acceptable.

Having thus described my invention in detail and with sufiicient particularity as to enable those skilled in the art to practice it, what is desired to have protected by Letters Patent is set forth in the following claims.

I claim:

1. Ceramically bonded brick made from a size-graded brickmaking batch having a coarse (plus 65 mesh) and a fine (minus 65 mesh) fraction comprised of fused magnesite-chrome ore grains, comprising the grains in the fines fraction having a substantially different magnesite: chrome ore ratio than the grains in the coarse fraction, the magnesia content of the brick being between and 75 percent, by weight, and a silica content of not greater than 3 percent.

2. Brick according to claim 1 in which the chrome ore content of the fines fraction is a least 10 percent different from the chrome ore content of the coarse fraction.

3. Brick according to claim 1 in which the fines fraction is selected from the group consisting of: (1) fused grains having magnesitezchrome ore ratios in excess of 55:45, and (2) fused grains having magnesite:chrome ore ratios less than 45:55 and said coarse fraction selected from the same group but different than the fines fraction.

4. Brick according to claim 1 in which the batch is size-graded so that from 25 to 50 percent passes minus mesh and at least 40 percent is greater than 28 mesh.

5. Brick according to claim 1 in which the coarse fraction comprises magnesite-chrome ore grain having a magnesite:chrome ore ratio in excess of 55 :45.

6. Brick according to claim 1 in which the coarse fraction comprises magnesite-chrome ore grain having a magnesitetchrome ore ratio below 45:55.

7. Brick according to claim I in which the magnesia content of the brick is between 60 and percent by weight.

References Cited UNITED STATES PATENTS 3,297,457 11/1967 Charvat 106-59 JAMES E. POER, Primary Examiner 

